1. Field of the Invention
The present invention relates, in general, to a method for preparing an ester compound and, more specifically, to a method for preparing an ester compound by reacting a liquid alcohol with a liquid organic acid in the presence of lipase without adding any solvent.
Here, "ester compound" refers, for example, to ethyl, n-propyl, isopropyl, n-butyl,, iso-butyrated acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, iso-valeric acid or the like, all of which are used as natural flavors.
2. Description of the Prior Art
Usually, an ester compound is synthesized by chemical or biological methods. In the latter method, for example, lipase an esterase is used for reaction of an alcohol with a carboxylic acid to produce an ester compound and water.
In Biotech. Lett., 12, 581 (1990), Langrand et al., discloses the synthesis of an ester compound, wherein ethanol is reacted with n-butyric acid to produce ethyl butyrate and water as shown in the following formula: EQU Ethanol+n-butyric acid.fwdarw.ethyl butyrate+water
This formula can be generally represented as follows: EQU Alcohol+carboxylic acid.fwdarw.ester compound+water
The ester compound can also be prepared chemically. However, the preference to natural compounds induces researchers to biologically synthesize ester compounds using fermented materials of microorganisms or enzymes. For example, it is reported in a great number of literatures that ester compounds are prepared in an aqueous state (Armstrong et al., Biotechnol. Bioeng. 26, 1038 (1984); Williams et al., Ann. New York Acad. Sci., 542, 406 (1988); Murray and Duff, Appl. Microbiol. Biotechnol. 33. 202 (1990); Fukuda et al., Curr, Genet. 20, 49 (1991); Fukuda et al., J. Ferm. Bioeng. 75, 288 (1993)). These active researches demonstrated the production of ester but have a disadvantage in that the ester compounds produced are extremely low in solubility in water.
Other reaction systems than water include solvent phase (Carta et al., Biotechnol. Bioeng. 37, 1004 (1991); Langrand et al., Biotech. Lett. 12, 581 (1990); Carta et al., Enzyme Microb. Technol., 14, 904 (1992)), supercritical fluid phase (Marty et al., Biotechnol. Bioeng., 39, 273 (1992); Marty et al., Biotechnol. Bioeng., 43, 497 (1994)) and gas phase (Hwang and Park, Biotech. Lett., 16, 379, (1994)). However, the solvent phase reaction is disadvantageous in that a large amount of cost is required to separate and collect the product from the reaction system (solution). The application of the supercritical fluid phase reaction or the gas phase reaction for an industrial process scale is still in a beginning stage. Accordingly, using only liquid phase substrates in the absence of solvent can bring about the advantages of reducing production cost, being safe and facilitating separation and collection. Thus, the natural flavors can be produced on a large scale in this manner (see: Carta et al., Enzyme Microb. Technol., 14, 904 (1992); Oguntimein et al., Biotech. Lett., 17, 77 (1995)).